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> > Hello - > > I just got back into ham radio from a long layoff (1981). Around 1982 I did > graduate work at the MSEE level and took quite a few courses in > electromagnetic theory and antennas. In those courses I don't remember any > references to linear loading. > > What is linear loading? After reading both the ARRL antenna handbook and > ON4UN's low band dxing book it appears that linear loading is some black > magic technique to make an antenna mechanically shorter. Are there any > references available that discuss this both intuitively and theoretically so > I could have a better feel on what is going on? Is linear loading the > accepted name in academia, or is it referred to by some other name? > > Secondly, how do the designers at F12 and other companies model the linear > loading circuits? Do they model it as discrete circuit components and throw > in some 'fudge' factors, or is there some analysis codes that can handle > this situation? Boy that would be fun modeling my c4sxl with the electric > field integral equation (EFIE) from scratch. I think I'll get right on that. > > 73s > > Bill, w6qd Hi Bill, Linear loading, as commonly used in amateur radio antennas, is nothing more than an inductor that is built from a shorted length of parallel wire (or rod) transmission line. I have a homebrew 45' long 40m rotatable dipole that uses linear loading. MININEC-3 (freeware) calculated the feedpoint impedance to be around 22 -j300 ohms with the antenna at 93' over perfect ground. I knew that I needed +j300 ohms of inductance to bring the antenna to resonance. Half of 300 is 150, so each inductor needed to be +j150 ohms. I built a parallel transmission line from 1/2" and 3/8" telescoping aluminum tubing, spaced about 3" apart. I forget what the characteristic Z of this line was... doesn't really matter. I also forget the formula that I used to determine where the line needed to be shorted to give me +j150 ohms at 7.050 MHz.... I'm sure that you can find it in your textbooks or the ARRL Antenna Handbook. In practice, I only needed a small amount of tweaking (+/- an inch) from what the formula predicted to achieve resonance. I now had a resonant antenna with a feedpoint Z of 22 ohms. I stepped this up to 50 ohms with the use of a helical hairpin match. It turned out to be 3 or 4 turns, about 4" or so in diameter, of 1/4" copper tubing. The feedpoint bolts on the dipole are about 3.5" apart, so that is how long the hairpin inductor is. I feed it with a 1:1 current balun. Mounted 93' in the air, it appears to perform quite well. Hope this helps. 73 es gl, Charlie N9CO -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. Force 12 Web site: http://www.qth.com/force12 Submissions: send to Force12Talk@qth.com To unsubscribe: send a blank e-mail to Force12Talk-leave@qth.com Force12Talk Message Archive: http://www.qth.com/force12/list/force12talk For problems with the list, contact n4zr@qth.com |
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